Enhanced removal of selenate from mining effluent by HO/HCl-pretreated zero-valent iron.

Direct use of zero-valent iron (ZVI) in reductive removal of selenate (Se(VI)) is inefficient due to the intrinsic passive layer of ZVI. Here we observed that ZVI pretreated with HO (P-ZVI-O) performs much better in Se(VI) removal from a mining effluent than other three modes of ZVI alone, acid washing ZVI (P-ZVI-A), and simultaneous addition of HO and ZVI (ZVI-O) as well. The P-ZVI-O exhibits exceptionally high Se(VI) removal at a low dosage, wide pH range, with Se dropping down from 93.5 mg/L to <0.4 μg/L after 7-h reaction. Interestingly, the initial pH (2-6) of the mining effluent exerted little influence on the final Se(VI) removal. HO/HCl pretreatment results in the formation of various reducing corrosion products (e.g. FeO, FeO and Fe), which greatly favors the efficient Se(VI) removal. In addition, surface-bound Fe ions participated in the reduction of Se(VI). Combined with the influence of Se valence as well as pH and Fe (whether dissolved or surface bound), it is deduced that the P-ZVI-O mode induced efficient Se(VI) removal via the adsorption-reduction and/or co-precipitation. This study demonstrates that HO/HCl pretreatment of ZVI is a very promising option to enhance the efficiency of reductive removal of Se(VI) from real effluents.